The use of membrane bioreactors (MBR) and filtration membrane modules for treating raw water or wastewater is known in principle. The membranes used for filtration consist, for example, of polymeric materials such as polyethylene, polypropylene, polyethersulfone, polyvinylidene fluoride or similar polymers. The pore sizes of the membranes are for these uses in the range between 0.001 and 1 μm. In a membrane bioreactor, the activation method for wastewater treatment with separation of the biomass from the purified water is carried out using ultra- or microfiltration membranes. In most applications, the polymer membranes are immersed directly in the activated sludge and the treated wastewater is drawn off by means of vacuum suction or flows off under the influence of gravity.
In the MBR method, the wastewater is physically, chemically and biologically treated in a plurality of steps until it reaches the membrane. By means of mechanical and physical pretreatments, the wastewater is freed from particles, fibers and coarse matter. In the coarse filtration, large particles which could cause damage to the membranes are removed by grills and screens. In the MBR method, fine screens in a size range of 0.05-3 mm are customarily used as prefiltration. The wastewater is in addition freed from heavy particles (e.g. sand) and oils and fats by a sand and fat trap.
In a further treatment step of the wastewater treatment, the wastewater is biologically and chemically treated. In the activation tank there is situated the activated sludge (biomass) which contains in its biomass the enzymatic potential for conversion of the high-molecular-weight pollutants in such a manner that these can be eliminated. The dissolved materials are utilized by the biomass either for the cell structure or for energy production with oxygen consumption. The resultant oxygen consumption must be covered by sufficient oxygen supply, for which reason activation tanks are provided with aeration appliances. A precondition for the functioning of the method is that the biomass remains in the system. Therefore, the biomass is separated off from the purified wastewater by membrane filtration and recirculated to the activation tank. Overgrown activated sludge is removed as excess sludge. Before the biomass is separated from the water, further chemical treatments proceed. In combination with a filtration stage, various precipitants and flocculants such as, for example, iron chloride or polymers for removing colloidally and particulately dispersed liquid components are customarily used.
A substantial advantage of MBR systems is the solids-free effluent. This means, in addition, that no bacteria are found in the effluent of the membrane activation system and, even viruses may be separated off by sorption effects. The residual organic pollution is reduced thereby owing to the complete separation. The hygienically relevant guide values of the EU bathing water directive [75/160/EEC, 1975] are complied with using MBR. In addition, the solids-free effluent offers not only in the municipal sector but also in the industrial sector, a great potential for wastewater reuse. Here, by water recycling up to closed circulation of water large savings of water can be achieved. A further advantage is that in this method, owing to the adjustable high DM content and the omission of the clarifiers, only a very small space is required. Owing to the independence of the sedimentation behavior, the activated sludge concentration (biomass concentration, expressed as DM—dry matter) can be increased over conventional methods. Membrane bioreactors are customarily operated at DM concentrations of 8 to 15 g/l. Compared with the conventional activation method, the reactor volume of a membrane bioreactor can be reduced, in such a manner that higher volumetric loadings are possible.
In the case of the membrane bioreactor method which is based generally on the aerobic activation method that is combined with a membrane filtration unit, the biomass is recirculated as concentrate over the membrane filtration unit, while the purified water is separated off as filtration permeate.
A problem in the use of membrane filters in the field of wastewater purification is what is termed “membrane fouling”, which means that deposits form on the membranes, which deposits decrease the through-flow of the liquid that is to be purified.
DE 102 20 916 A1 describes a filtration appliance and also a membrane bioreactor which are operated under conditions in the filtration medium such that membrane fouling and deposits on the membrane surfaces are reduced. For this purpose the filtration device has hollow fiber membranes that are combined to form a fiber bundle for separating off the particles from a liquid, through which hollow fiber membranes liquid flows from the outside to the inside, and the filtered liquid is then taken off from at least one of the ends of the hollow fiber membranes. The filtration device, in addition, has a gas feed device in order to flush a gas over the exterior of the hollow fiber membranes. The fiber bundle in this case is wound round the outer peripheral surface of a carrier of the gas feed device.
EP 1 734 011 A1 (whose United States equivalent is United States Patent Publication No. 2006/272198A1) discloses a method for improving the flow through a membrane bioreactor, in which a certain fraction of cationic, amphoteric and zwitterionic polymers or a combination thereof is added. The fraction of the added polymers is 10 to 2000 ppm, based on the entire membrane bioreactor volume. The polymers have a molecular weight of 10 000 to 20 000 000 Da. Adding the abovementioned polymer should reduce, especially inorganic fouling, which is formed by the precipitation of limestone CaCO3 onto the membrane surfaces from the wastewater that is to be purified. The pH increases in the course of this, whereby in turn the precipitation is promoted by calcium phosphate and iron oxide. The precipitation of carbonates and phosphates in the wastewater proceeds in the form of small particles which are retained on the membrane surfaces.
Quite generally it is true that membrane fouling due to the precipitation of bioactive solids, colloids, accumulation of particles or macromolecular particles on the membrane surface leads to a decrease in the through-flow and permeability. It is difficult to describe the fouling process exactly owing to the heterogeneity of the activated sludge. Factors such as characteristics of the biomass, the extracellular polymeric substance, pore size, surface characteristics and membrane material, and also the construction of the filter membrane modules and the operating conditions influence fouling growth. For example, biofouling occurs most frequently on nanofiltration and reverse osmosis. The reason is that the membranes cannot be disinfected with chlorine in order to kill bacteria. The biofouling is principally due to the complex growth behavior of the bacteria. The type of microorganisms, the growth rate thereof and concentration on the membranes depend chiefly on the critical factors such as temperature, pH, the concentration of dissolved oxygen and the presence of organic and inorganic nutrients. It should be noted that the microorganisms pass into the filtration systems via air and/or water.
In the case of the filtration methods using membrane bioreactors, the growth of the fouling is customarily monitored in a plurality of steps.
1. Pretreatment of the raw water or wastewater, before inflow into the activated sludge, by means of various filtration steps as have already been mentioned above, for which purpose fine-mesh gratings having a mesh width of 0.5 to 3 mm are used.
2. In the “crossflow” method, the liquid that is to be purified is circulated along the membrane surface, for which purpose in the case of submerged modules, aeration devices are installed below the membrane modules, which aeration devices induce an upward streaming.
3. In some membrane modules a regular fully automatic backwash with permeate is performed, in such a manner that adhering particles/dirt are detached from the membrane surface and the pores are flushed open. A precondition is that the respective membrane is backwashable.
4. Chemical cleaning: the steps serve to prevent membrane fouling or at least decrease it. Chemical cleaning is necessary in order to remove the membrane fouling layers on and within the membranes. Chemical cleaning gives rise to considerable operating costs, since during the cleaning the membranes are out of operation and therefore additional membranes must be installed.
In addition, it is disadvantageous that the chemicals used such as, for example, sodium hydrochlorite NaOCl adversely affect the environment and contribute to the formation of absorbable organic halogen compounds (AOX). In addition, chemical cleaning requires an additional infrastructure (pumps, chemical containers, leak measuring devices, protective equipment, . . . ) which is costly. Frequently, the membranes are chemically cleaned in a separate cleaning container in order to save chemicals, since these cleaning containers have small volumes. For this purpose the membrane module must be taken out of the filtration pond or tank and installed in the cleaning pond or tank. In the cleaning pond/tank, the chemical cleaning then takes place. The operating personnel must be trained to handle these chemicals and chemical cleaning is labor-intensive. Overall, chemical cleaning is a considerable cost and environmental factor.
For avoiding fouling layers, the publication of the company VA TECH WABAG GmbH, Vienna, editor: F. Klegraf with the title “Beherrschung von Fouling und Scaling an getauchten Filtrationssystemen in Membranbelebungs-anlagen” [Managing fouling and scaling on submerged filtration systems in membrane activation systems] describes the use of abrasively acting inert inorganic porous materials which can detach deposits on the surface of the membranes by long-term action. This use is not uncontroversial, since it must be feared that the abrasive forces not only erode the deposits, but also damage the sensitive surfaces of the membranes. As inert abrasive material, expanded clay is mentioned which is introduced into the reactor. The expanded clay is retained in the reactor by screens. The turbulence introduced into the reactor with the flushing air is sufficient to homogenize the expanded clay in the system. Immediately after charging the reactor with expanded clay, the increase in filtration performance can be measured and by careful increasing of the expanded clay concentration in the activated sludge, 75% of the preset value of the filtration performance can be achieved after an experimental time of 40 days. Further increase of the expanded clay concentration in the reactor is not accompanied by any lasting improvement of the filtration results. The density of the porous expanded clay increases with time owing to water absorption. The expanded clay particles become heavier thereby and settle within the reactor and circulate only to a small extent as a result of the liquid streaming. In order to stimulate the circulation of the expanded clay particles, relatively large amounts of compressed air are then necessary but owing to the increased feed of compressed air into the liquids that are to be purified, other process parameters can be adversely affected thereby, for example maintaining preset theoretical oxygen values is made considerably more difficult. The velocity of ascension of the particles here is predetermined by the size of the air bubbles formed, but not by the amount of air introduced.